1. Field of the Invention
The present invention relates to a magnetic resonance tomography device and to a method for operating a magnetic resonance tomography device, particularly for executing more rapid sequences with high image resolution without triggering stimulations in a living examination subject.
2. Description of the Prior Art
MR tomography is a known technique for acquiring images of the inside of the body of a living examination subject. To this end, rapidly switched magnetic gradient fields of high amplitude, which are generated by gradient coils, are superimposed on a static basic magnetic field.
In the process of picking up MR images, stimulations can be triggered in living examination subjects by the switching of the gradient fields. The gradient fields that influence the examination subject are characterized by a magnetic flux density that varies over time. The time-varying magnetic field generates eddy or induction currents in the examination subject. Their nature depends primarily on the shape and size of the microscopic structures. Due to electromagnetic interaction with tissue in the examination subject, these currents influence physiological currents, for instance potentials at cells. All cells have a resting potential. At resting potential, all membrane currents of a cell are in balance. When the membrane potential is depolarized by an additional membrane current which is introduced into the cell by an outside influence, for example, this causes a potential change, known as an action potential. The actuating potential for an action potential is called a threshold. At the threshold, the balance of the membrane currents changes. Additional currents temporarily appear, which depolarize the membrane. An action potential is accompanied by an action. Thus, for example, each contraction of a muscle fiber is accompanied by an action potential in the muscle fiber, and each reaction of a sensory cell to a sensory stimulus is relayed by action potentials. Accordingly, due to the triggering of action potentials, switched gradient fields can lead to stimulations that are experienced as uncomfortable by the examination subject.
There are known methods for predicting these stimulations. One of these methods for monitoring stimulations is based on the so-called xe2x80x9cdB/dt modelxe2x80x9d. In this method the values, which occur in an MR tomography process, of the time variation of magnetic flux density of gradient fields (dB/dt values) are checked and monitored. The maximum allowable dB/dt values arise as a result of a stimulation study with the corresponding gradient coils, or from limit values that have been strictly prescribed by regulatory authorities. Further details can be found in (xe2x80x9cPeripheral Nerve Stimulation by Time-Varying Magnetic Fieldsxe2x80x9d (J. Abart, er al; J. Computer Assisted Tomography (1997) 21 (4):532-8).
Another known approach to stimulation monitoring is known as the xe2x80x9cIrnich modelxe2x80x9d. This method describes the stimulation threshold as a function of the duration of the external influence. The period of influence is the time in which the amplitude of the gradient field changes in one direction; that is, dB/dt is permanently  greater than 0 or  less than 0, respectively. A more detailed explanation can be found in the essay xe2x80x9cElectrostimulation By Time-Varying Magnetic Fieldsxe2x80x9d (W. Irnich, MAGMA (1994),2:43-49) and xe2x80x9cMagnetostimulation in MRIxe2x80x9d (W. Irnich, F. Schmitt, MRM (1995) 33:619-23).
Furthermore, German OS 42 25 592 teaches the prevention of stimulations of this sort by covering, with a closed conductor loop, regions of an examination subject outside the imaging volume that are sensitive to stimulation. This produces a reduction in the currents induced in the covered region. This method is based on the recognition that, given switched gradient fields, the highest current values are induced outside the imaging volume, and so the danger of stimulations is greatest there. Introducing conductor loops outside the imaging volume compromises the linearity of the gradient fields in the imaging region only inconsequently, which is important for image quality; however, given a change of the region of the subject that is to be imaged the position of the conductor loops must usually also be modified.
The triggering of stimulations thus depends basically on the type of measurement sequence. It is necessary to distinguish between sequences known as conventional measuring sequences and sequences known as rapid measuring sequences. Conventional measuring sequences usually demand a high linearity of the gradient fields within a definite linearity volume, for instance 5% in a linearity volume of 40 to 50 cm given moderate gradient strengths of 10 to 20 mT/m and switching times of approx. 1 ms. For the rapid measuring sequences, high gradients, for instance 20 to 40 mT/m, are switched very rapidly (switching times approx. 100 to 500 xcexcs). The time-varying magnetic flux density of the gradient fields induces electrical currents in the examination subject, which can trigger stimulations of the subject. With faster time variations, that is, faster switching times and larger values of magnetic flux density of gradient fields, the induced currents are greater, and the probability of stimulations increases. The largest values in absolute terms are attained at the margins and outside the linearity volumes, where the maximum field deviation or excursion also occurs. Given defined requirements on the size of the gradients and the switching time, the field deviation, and thus the risk of stimulation, can be reduced by using a gradient coil with a smaller linearity volume. Thus, in rapid measuring sequences, the linearity volume of typically 40 to 50 cm drops to 20 cm, for example. A gradient coil with the above described characteristics for rapid measuring sequences typically is not suitable for conventional whole-body applications, but rather for rapid MR imaging techniques such as EPI (described in U.S. Pat. No. 4,165,479) and what are known as turbospin methods, such as the GRASE and HASTE methods.
The German OS 195 40 746 describes a modular gradient coil system that unites, in one coil body, a gradient coil for rapid measuring sequences and an activatable gradient coil for conventional measuring sequences. The gradient coil for rapid measuring sequences has a small linearity volume and allows rapid switching of gradient fields with large gradients. In the joint operation of the two coils, the gradient coil system has a large linearity volume for conventional measuring sequences with slowly switched gradient fields and given small gradients. This has the disadvantage that, with the selection of a rapid or conventional measuring sequence, an imaging region is defined corresponding to the appertaining linearity volume. The imaging region for rapid measuring sequences is always a definite small subregion, which is strictly prescribed by the coil arrangement, of the larger imaging region for conventional measuring sequences, with the midpoint of the two imaging regions being identical. To pick up MR images with rapid measuring sequences for an imaging region extending over the imaging region for conventional measuring sequences, the examination subject would have to be moved in all three directions in space. Due to the geometry of the MR tomography device, however, it is only possible to shift the examination subject in one direction.
Furthermore, U.S. Pat. No. 5,311,135 teaches a gradient coil for a magnetic resonance device which has four saddle-shaped coils, each of which has first and second terminal points respectively at the beginning and end of its conductor, as well as at least one tapping point between the terminal points. The arrangement also includes a switching mechanism, so that each of the coils can be supplied with current either between the terminal points or between the first terminal point and the tapping point. In this way, at least two different linearity volumes of the gradient coils can be set, for instance corresponding to a size of a region that is being imaged, as in German OS 195 40 746.
An object of the present invention is to provide an MR tomography device and a method for operating an MR tomography device, wherein the above described disadvantages of known systems and methods are alleviated.
This object is inventively achieved in an MR tomography system having a gradient coil with at least two independently controllable portions, a coil control device which controls the gradient coil, namely to control the coil portions thereof, and wherein the coil control device controls the gradient coil portions in at least one first control state for generating a gradient field for a first imaging subregion, and the coil control device controls the gradient coil portions in at least one second control state for generating a gradient field for a second imaging subregion, which is not a subset of the first imaging subregion and which does not contain it.
The above object also is inventively achieved in a method for operating an MR tomography system including the steps of controlling a gradient field in a first imaging subregion by controlling coil portions of a gradient coil containing at least two independently controllable portions, and controlling a gradient field in a second imaging subregion, which is not a subset of the first imaging subregion and which does not contain it.
By controlling gradient fields for at least two imaging subregions, with neither of the two regions being a subset of the other, it is possible to pick up MR images for a larger aggregate imaging area, which derives at least from the sum of the two imaging subregions, using rapid, high-resolution measuring sequences without triggering stimulations. In particular, the MR image scans can be performed for a region of a subject which has center, relative to the spatial directions in which the subject is constrained from movement due to the geometry of the MR tomography system, that does not coincide with the center of the aggregate image region. This also applies with respect to the spatial directions in which the subject can be moved (using the displaceable patient bed of the MR device), which creates the advantage that it is not necessary to displace the subject for these MR image scans. The term xe2x80x9cgradient fieldxe2x80x9d as used herein is a magnetic field extending in space that has a constant magnetic flux density gradient at least in the region of an imaging region for which it is generated.
In an embodiment, the coil control device can also control (operate) the gradient coil portions in at least one third control state for generating a gradient field for an aggregate imaging region which is defined by the gradient field of maximal extent that can be generated by the gradient coil and which contains all imaging subregions. In this way, conventional measuring sequences also can be executed for an aggregate imaging region of maximal extent, and are not restricted to one of the imaging subregions.
In a further embodiment, the coil control device controls the coil portions in a control state in which at least one coil portion, which is not contributing (at the moment) to the generation of a gradient field for an imaging subregion, generates a magnetic field whose values for magnetic flux density are not equal to the values exhibited by the gradient field within the imaging subregion. Under the assumption that the MR signals are picked up by a high-frequency receiving antenna that is designed for the aggregate imaging region, this embodiment prevents signals from regions outside a desired imaging subregion from having the same frequencies as signals from the desired imaging subregion, which would lead to falsification of the image.
In another embodiment, a test related to the triggering of stimulations is conducted on a set measuring sequence prior to its execution for image data acquisition. From the results of this test, a sequence format is determined for executing the set measuring sequence without triggering stimulations by controlling, in chronological succession, gradient fields for the aggregate imaging region, for one of the imaging subregions, or for at least two of the imaging subregions; and the determined format of execution is implemented for image data acquisition.
The testing of stimulation triggering is accomplished using one of the above described methods of stimulation prediction, for example. With the setting of a measuring sequence, among other things its time sequence, and thus the time characteristic of the gradient field, is strictly prescribed. In a further specified imaging region for slices that are being imaged, this prevents magnetic flux density of a gradient field extending beyond the specified imaging region, particularly at the edges of the imaging region, from exceeding defined limit values. A maximum size of the gradient, which simultaneously determines the maximum image resolution of an MR image, is thus specified. If the set measuring sequence demands a higher image resolutionxe2x80x94i.e. a larger gradientxe2x80x94than can be achieved with the stimulation-related limit values, the measuring sequence can be executed with the desired image resolution while maintaining its time sequence, but for a smaller imaging region, since, given a smaller imaging region and the same limit values, it is possible to set a larger gradient without triggering stimulations.
In the context of the last described embodiment, this means that if, for a set measuring sequences, stimulations are expected given gradient fields that extend beyond the aggregate region, the set measuring sequence is executed with the desired image resolution with gradient fields for imaging subregions. An attempt is made to select an imaging subregion in such a way that all the set slices that are being imaged fall into this subregion. If the set slices being imaged extend over a broad area, it may be necessary to set gradient fields of corresponding gradient sizes for at least two imaging subregions in chronological succession. To do this, in a further embodiment the coil control device is designed to control operation so that a gradient field is initially set up in a first imaging subregion and then in a second imaging subregion in chronological succession.